java - ViewPagerAdapter 显示空值

Question

我正在尝试在我的应用程序中实现 Material Design 中的滑动选项卡。找了个教程，照着做了代码。我做了一些更改以在我的应用程序上实现。现在我得到一个 NullPointerException 并且无法找出原因。

这是我遵循的教程

http://www.exoguru.com/android/material-design/navigation/android-sliding-tabs-with-material-design.html

我的代码

public class Dashboard extends Fragment {

    ViewPager pager;
    ViewPagerAdpater adapter;
    SlidingTabLayout tabs;
    CharSequence Titles[]={"Songs","Videos","Featured","Radio","Playlist","Artists"};
    int Numboftabs =6;

    @Override
    public View onCreateView(LayoutInflater inflater,  ViewGroup container,  Bundle savedInstanceState) {
        View view =inflater.inflate(R.layout.activity_slide,container,false);

        adapter =  new ViewPagerAdpater(getParentFragment().getFragmentManager(),Titles,Numboftabs);


        // Assigning ViewPager View and setting the adapter
        pager = (ViewPager) getView().findViewById(R.id.pager);
        pager.setAdapter(adapter);

        // Assiging the Sliding Tab Layout View
        tabs = (SlidingTabLayout) getView().findViewById(R.id.tabs);
        tabs.setDistributeEvenly(true); // To make the Tabs Fixed set this true, This makes the tabs Space Evenly in Available width

        // Setting Custom Color for the Scroll bar indicator of the Tab View
        tabs.setCustomTabColorizer(new SlidingTabLayout.TabColorizer() {
            @Override
            public int getIndicatorColor(int position) {
                return getResources().getColor(R.color.tabsScrollColor);
            }
        });

        // Setting the ViewPager For the SlidingTabsLayout
        tabs.setViewPager(pager);

        return view;
    }
}

这里我得到一个空指针异常

adapter =  new ViewPagerAdpater(getParentFragment().getFragmentManager(),Titles,Numboftabs);

ViewPagerADapter

public class ViewPagerAdpater extends FragmentStatePagerAdapter {

    CharSequence Titles[]; // This will Store the Titles of the Tabs which are Going to be passed when ViewPagerAdapter is created
    int NumbOfTabs; // Store the number of tabs, this will also be passed when the ViewPagerAdapter is created


    // Build a Constructor and assign the passed Values to appropriate values in the class
    public ViewPagerAdpater(FragmentManager fm,CharSequence mTitles[], int mNumbOfTabsumb) {
        super(fm);

        this.Titles = mTitles;
        this.NumbOfTabs = mNumbOfTabsumb;

    }



    //This method return the fragment for the every position in the View Pager
    @Override
    public Fragment getItem(int position) {

        if(position == 0) // if the position is 0 we are returning the First tab
        {
            SongsFragment tab1 = new SongsFragment();
            return tab1;
        }
        else if(position == 1)            // As we are having 2 tabs if the position is now 0 it must be 1 so we are returning second tab
        {
            ArtistsFragment tab2 = new ArtistsFragment();
            return tab2;
        }
        else{

            Dashboard tab3 = new Dashboard();
            return tab3;
        }

    }

    // This method return the titles for the Tabs in the Tab Strip

    @Override
    public CharSequence getPageTitle(int position) {
        return Titles[position];
    }

    // This method return the Number of tabs for the tabs Strip

    @Override
    public int getCount() {
        return NumbOfTabs;
    }
}

主要 Activity

public class MainActivity extends ActionBarActivity
        implements NavigationDrawerCallbacks {

    /**
     * Fragment managing the behaviors, interactions and presentation of the navigation drawer.
     */
    private NavigationDrawerFragment mNavigationDrawerFragment;
    private Toolbar mToolbar;
    public static FragmentManager fragmentManager;


    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        mToolbar = (Toolbar) findViewById(R.id.toolbar_actionbar);
        setSupportActionBar(mToolbar);
        getSupportActionBar().setDisplayShowTitleEnabled(false);
        fragmentManager = getSupportFragmentManager();

        mNavigationDrawerFragment = (NavigationDrawerFragment)
                getFragmentManager().findFragmentById(R.id.fragment_drawer);

        // Set up the drawer.
        mNavigationDrawerFragment.setup(R.id.fragment_drawer, (DrawerLayout) findViewById(R.id.drawer), mToolbar);


    }

    @Override
    public void onNavigationDrawerItemSelected(int position) {
        // update the main content by replacing fragments
        // Toast.makeText(this, "Menu item selected -> " + position, Toast.LENGTH_SHORT).show();
        FragmentManager fragmentManager = getSupportFragmentManager();
        fragmentManager.beginTransaction()
                .replace(R.id.container, new Dashboard()).commit();
    }


    @Override
    public void onBackPressed() {
        if (mNavigationDrawerFragment.isDrawerOpen())
            mNavigationDrawerFragment.closeDrawer();
        else
            super.onBackPressed();
    }


    @Override
    public boolean onCreateOptionsMenu(Menu menu) {
        if (!mNavigationDrawerFragment.isDrawerOpen()) {
            // Only show items in the action bar relevant to this screen
            // if the drawer is not showing. Otherwise, let the drawer
            // decide what to show in the action bar.
            getMenuInflater().inflate(R.menu.main, menu);
            return true;
        }
        return super.onCreateOptionsMenu(menu);
    }


    @Override
    public boolean onOptionsItemSelected(MenuItem item) {
        // Handle action bar item clicks here. The action bar will
        // automatically handle clicks on the Home/Up button, so long
        // as you specify a parent activity in AndroidManifest.xml.
        int id = item.getItemId();

        //noinspection SimplifiableIfStatement
        if (id == R.id.action_settings) {
            return true;
        }

        return super.onOptionsItemSelected(item);
    }


}

LOGCAT

Process: com.example.crytal.etunesdev, PID: 7698
    java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.crytal.etunesdev/com.example.crytal.etunesdev.MainActivity}: java.lang.NullPointerException
            at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2404)
            at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2464)
            at android.app.ActivityThread.access$900(ActivityThread.java:172)
            at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1308)
            at android.os.Handler.dispatchMessage(Handler.java:102)
            at android.os.Looper.loop(Looper.java:146)
            at android.app.ActivityThread.main(ActivityThread.java:5653)
            at java.lang.reflect.Method.invokeNative(Native Method)
            at java.lang.reflect.Method.invoke(Method.java:515)
            at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1291)
            at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1107)
            at dalvik.system.NativeStart.main(Native Method)
     Caused by: java.lang.NullPointerException
            at com.example.crytal.etunesdev.Dashboard.onCreateView(Dashboard.java:28)
            at android.support.v4.app.Fragment.performCreateView(Fragment.java:1789)
            at android.support.v4.app.FragmentManagerImpl.moveToState(FragmentManager.java:955)
            at android.support.v4.app.FragmentManagerImpl.moveToState(FragmentManager.java:1138)
            at android.support.v4.app.BackStackRecord.run(BackStackRecord.java:740)
            at android.support.v4.app.FragmentManagerImpl.execPendingActions(FragmentManager.java:1501)
            at android.support.v4.app.FragmentActivity.onStart(FragmentActivity.java:551)
            at android.app.Instrumentation.callActivityOnStart(Instrumentation.java:1177)
            at android.app.Activity.performStart(Activity.java:5551)
            at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2377)
            at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2464)
            at android.app.ActivityThread.access$900(ActivityThread.java:172)
            at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1308)
            at android.os.Handler.dispatchMessage(Handler.java:102)
            at android.os.Looper.loop(Looper.java:146)
            at android.app.ActivityThread.main(ActivityThread.java:5653)
            at java.lang.reflect.Method.invokeNative(Native Method)
            at java.lang.reflect.Method.invoke(Method.java:515)
            at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1291)
            at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1107)
            at dalvik.system.NativeStart.main(Native Method)

Answer 1

在实例化 ViewPagerAdpater 时摆脱 getParentFragment()。 fragment 的父级是 Activity，当您处理嵌套 fragment 时，getParentFragment() 返回一些东西 != null。

还有，

pager = (ViewPager) getView().findViewById(R.id.pager);。

getView() 返回结果 onCreateView，因此您不能在 onCreateView 中使用它。使用膨胀 View (在您的情况下为 view)调用 findViewById。